Exhaust gas sampling systems that utilize partial flow dilution tunnels (PFDT) have been used since the early 1990's as an effective alternative to full dilution tunnels systems for development and certification of engines where steady-state model testing was permitted. Previously, all off-highway and until recently, much of European-bound on-highway engine testing and certification was performed with systems utilizing PFDT's, due to the fact that they are more portable, cheaper and more repeatable than their full dilution counterparts. Regulatory organizations such as ISO, CARB, EPA and EEC all permit the use of PFDT's for steady state test cycle certification. One such system is disclosed in U.S. Pat. No. 5,058,440 issued on Oct. 22, 1991 to Russell R. Graze and is assigned to the owner of the present application.
The Environmental Protection Agency has declared its interest in promulgating transient cycle regulations of large off-highway diesel engines in order to better control particulate emissions output from these engines. These regulations are expected to be in effect by 2006. The size of the off-highway diesel engines to be regulated eclipses the mass flow rate capacity of the industry's full dilution tunnels that have been in use for the past twenty plus years to quantify on-highway engine emission levels including particulate matter (PM). Furthermore, the sheer number of off-highway ratings to be developed, in combination with concurrent regulatory pressures placed on on-highway engine development teams, preclude the use of existing full dilution tunnels for off-highway development even for small engines.
Therefore it is desirable to develop a PFDT that can be used to test and certify off-highway diesel engines under transient conditions and more than likely will be utilized to test on-highway engines under transient conditions as well. One such system is described in U.S. Pat. No. 6,062,092 issued to Christopher Weaver on May 16, 2000 and is assigned to Engine, Fuel, and Emissions Engineering, Incorporated. This system utilizes a feedback arrangement to change the proportion of exhaust gas being sampled based on changes in the exhaust flow from an engine. However, this system uses the pressure differential between the exhaust gas flow stream and the pressure inside a sampling probe for feedback to control the proportionality of the exhaust sample and diluent air in a dilution tunnel. This system uses the last possible point in a test system to check for excursions in engine gas flow to sample for test changes (i.e. the exhaust gas flow stream) and does not discuss or take into account the pneumatic capacitance of the system. Pneumatic capacitance is the resistance in the sampling system to react to changes in system under transient conditions. Additionally, the U.S. Pat. No. 6,062,092 prior art does not take into account particulate deposition which is caused by thermophoresis.
The present invention is directed to overcome one or more of the problems as set forth above.